1. Field of the Invention
The present invention relates to an ink-jet ink and an ink-jet printing method. It also relates to an ink set, ink cartridge, printing unit and Ink-jet printing apparatus. The present invention is applicable for any apparatus that can print on a printing medium such as printing paper, cloth, leather, OHP paper and nonwoven fabric, especially applicable to business equipments such as printers, copiers and facsimiles utilizing ink-jet printing process.
2. Related Background Art
The Ink-jet printing method has various advantages including low noise operation, low running cost, high-speed printing, and easy adaptation for down-sizing and for color printing. Thus, ink-jet printing is widely employed for printers, copying machines, or the like. In general, the ink employed for printers is selected in consideration of its printing properties such as ejecting properties and fixing properties and print qualities such as bleeding, optical reflection density, and coloring of the printed images.
As is well known, inks are divided into two types according to the coloring material contained in the ink, i.e., the dye ink and the pigment ink. The pigment ink has various advantages in comparison with the dye ink, such as superior water resistance and light fastness, and clear character printing.
The pigment in a pigment ink is generally stably dispersed in the ink by the electric repulsion force of the polymer dispersant which destroys the agglomeration of the pigment particles due to intermolecular force. Therefore, it is preferable to add a polymer dispersant to an ink in an amount according to the amount of the pigment.
When such a pigment ink is applied onto a recording medium such as plain paper by an ink-jet printing process for printing characters, the solvent, e.g. water, of the ink penetrates into the paper and evaporates into the ambient air, and the pigment particles agglomerate. In this case, the more a polymer dispersant is added, the stronger the agglomeration force of the ink on the paper becomes. Thus, when an Ink dot is formed on a paper sheet with a prescribed volume of the ink ejected from an ink-jet head, the diameter of the dot is small and the dot shape remains irregular due to the impact of landing on the paper. Therefore, in order to obtain an ink dot having a sufficient printing density and a dot size necessary for forming a printing image free from white stripes or other similar defects, the ejection volume of an ink from an ink-jet head must be adjusted rather high. Even with such an adjustment, the presence of a polymer dispersant in a pigment ink, in combination with the penetrability decrease of the ink due to the strong agglomeration force of the pigment particles adsorbed on the dispersant, may delay the fixation of a pigment ink on a printing medium or lower the rub-off resistance of the recorded image.
In order to make the dot size larger and to improve the fixation properties, a penetrant may be added to a pigment ink to enhance penetration of the ink into a printing medium. However, use of a penetrant may cause undesirable phenomena such as the irregular peripheral shape of the dot (feathering), and penetration of the ink to the back of the printing medium (back-through), which are undesirable for the high quality recorded image. Further, since the coloring material penetrates into the printing medium, it often occurs that the optical density (OD) of the ink dot is not so much increased as the dot size increases.
To solve such problems, inks containing a self-dispersing pigment have been proposed. Such an ink can provide dots of a larger size, since the pigment agglomeration force on the paper is weaker than that in conventional pigment inks in which the pigment is dispersed by a dispersant. It, however, is not sufficient yet.
As mentioned above, there is still much to study so as to develop a printing ink that satisfies various factors that affect the quality of printing such as fixability of the ink, enlargement of the ink dot size, uniform density in an ink dot, and high optical density of the ink itself, and further satisfies stability as an ink, especially ejection stability as an ink-jet ink. The same is said for the printing method.